Interactive lesson

Fruit vinegar, step by step

Interactive lesson: from apple juice to a bottle of clear vinegar: two fermentations, containers, the mother, measuring strength and what home-made vinegar is for.

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The whole lesson as text

1. Two fermentations

Vinegar forms in two fermentations that must not be mixed up.

First fermentation: alcohol — Yeasts eat the sugar in the juice and make alcohol and carbon dioxide. They don't need air, and fermentation takes 1–4 weeks.
Second fermentation: acid — Acetic acid bacteria turn the alcohol into acetic acid. They need air, so they work at the surface of the liquid, for 4–12 weeks.
The mother on the surface — A jelly-like skin forms on the surface: the mother (mother of vinegar). Old mothers sink to the bottom and a new one forms above.
Warm and dark — Acetic fermentation works best at 20–30 °C (68–86 °F), in the dark. Below 15 °C (59 °F) it almost stops.

Why: Yeasts turn sugar into alcohol without air, and acetic acid bacteria turn alcohol into acid with air. If you mix up the two fermentations, usually neither works.

Too much alcohol, more than 10–12 %, stops the acetic acid bacteria.
Juice with roughly 10–12 % sugar gives vinegar of roughly 4–6 % acidity.

2. Containers and place

Vinegar is acidic and dissolves metals, so the container matters.

Good containers — Glass, ceramic with a lead-free glaze, a clean small oak cask or quality food-grade stainless steel.
Never metal — Never aluminium, copper, iron or galvanised metal. Vinegar dissolves metals, and that means poisoning.
No old crocks — Acid draws lead out of old glazes. If you don't know where the glaze came from, don't use the crock for vinegar.
Muslin in two layers — Cover the container with muslin or tightly woven cloth in two layers and tie it down with an elastic band or string, against fruit flies.

Why: Within a few weeks vinegar draws lead out of an old glaze. Lead poisoning is slow and invisible, and does most harm to children and pregnant women.

Use a crock with a glaze of unknown origin for flowers or dry goods, not for vinegar, fruit butter or pickles.
Keep vinegar away from the bin and the compost caddy, because of fruit flies.